The invention relates generally to methods and apparatus for controlling generators such as are often used on engines. More particularly, the invention relates to generator control for connecting the generator to an energized bus.
Generators are commonly used as starters and electrical power plants on engines such as aircraft gas turbine engines, for example. In some applications, multiple generators are used for supplying electrical energy to a load. As a starter/generator for an engine, the generator armature is supplied electrical energy typically from a starter battery. The generator develops substantial torque initially to begin turning the engine. In a typical DC shunt generator, the armature current also is used to supply the field excitation current.
Eventually, the starter/generator and engine reach a sufficient speed for full engine start to begin. At such time, the battery is disconnected from the generator and the generator is thereafter connected to a main bus or load, typically through closure of a contactor switching mechanism sometimes referred to as a line contactor. After connection to the bus, the generator functions as a power source driven by the running engine. This switching from a starter to a generator is commonly referred to as starter cutoff. In addition to wanting adequate engine speed at starter cutoff, adequate generator speed is also needed to permit proper voltage regulation of the generator output, as well as to assure that the generator can accommodate the expected load after it is connected to the main bus.
After starter cutoff, the generator continues to increase in speed driven by the engine. Depending on the particular generator and expected load conditions, a typical starter cutoff speed may be about 5500 rpm while the full load generator speed may be 8000 rpm, for example. Closure of the line contactor to connect the generator to a bus can be controlled, for example, by the use of a generator speed sensor, or by comparing the generator open circuit voltage with the bus voltage. In the latter case, the line contactor may be closed when the generator output voltage reaches a predetermined threshold close to the bus voltage.
In cases where the bus is energized by a second generator, a paralleling function or load balancing may be included in a system operation.
However, systems that include such paralleling functions tend to exhibit transient disturbances when a generator is connected to an energized bus. For example, assume a first generator is already connected to a bus and thus energizing the bus. If a second generator is connected to the energized bus before the second generator reaches sufficient speed for regulation or load capacity, the paralleling function will tend to drop the output voltage of the first generator, at a time when that first generator is really the only one of the two at sufficient speed to handle the load and regulation. The slow speed generator may also experience undesirable reverse currents, which currents may cause a reverse current trip to disconnect the generator from the bus. As soon as this happens the generator output voltage will rise again to the threshold needed for connection to the bus and the line contactor will again close. This undesirable line contactor cycling may go on during the spin-up and produce considerable system disturbance.
Although a generator speed sensor can be used as a control function for determining when to close the line contactor, such sensors tend to be expensive high maintenance items. Furthermore, if a speed sensor becomes inoperative or its output is otherwise lost as an input to the generator control circuit, the entire start-up operation may be unnecessarily aborted.
The objectives exist therefore for a method and apparatus for controlling connection of a generator to an energized bus such that the generator speed is adequate for regulation and load sharing, independent of the need for a generator speed sensor.